So apparently, kyptonite advertises it as having the portability of a U-lock but the strength of a chain lock. Which is actually better?

Depends on what kind of tooling your friendly neighborhood bike thief carries with him.

I use this

ABUS Folding Lock 6500/85 black (55160)

These are also no nonsense..

ABUS Lock-Chain Combination 1010/110 black (18898)

3rd category armored cable locks , segmented steel sleeve over the cable inside.
ABUS 1025/100 (18907)